1. Field of the Invention
This invention relates to an electron-beam generating device having a plurality of matrix-wired cold cathode elements and to a method of driving the device. The invention further relates to an image forming apparatus to which the electron-beam generating device is applied, particularly a display apparatus using phosphors as image forming members.
2. Description of the Related Art
Two types of elements, namely thermionic cathode elements and cold cathode elements, are known as electron emission elements. Examples of cold cathode elements are surface-conduction electron emission elements, electron emission elements of the field emission type (abbreviated to xe2x80x9cFExe2x80x9d below) and metal/insulator/metal type (abbreviated to xe2x80x9cMIMxe2x80x9d below).
An example of the surface-conduction electron emission element is described by M. I. Elinson, Radio. Eng. Electron Phys., 10, 1290, (1965).
There other examples as well, as will be described later.
The surface-conduction electron emission element makes use of a phenomenon in which an electron emission is produced in a small-area thin film, which has been formed on a substrate, by passing a current parallel to the film surface. Various examples of this surface-conduction electron emission element have been reported. One relies upon a thin film of SnO2 according to Ellinson, mentioned above. Other examples use a thin film of Au [G. Dittmer: xe2x80x9cThin Solid Filmsxe2x80x9d, 9.317 (1972)]; a thin film of In2O3/SnO2 (M. Hartwell and C. G. Fonstad: xe2x80x9cIEEE Trans. E.D. Conf.xe2x80x9d, 519 (1975); and a thin film of carbon (Hisashi Araki, et al: xe2x80x9cShinkuuxe2x80x9d, Vol. 26, No. 1, p. 22 (1983).
FIG. 1 is a plan view of the element according to M. Hartwell, et al., described above. This element construction is typical of these surface-conduction electron emission elements. As shown in FIG. 1, numeral 3001 denotes a substrate. Numeral 3004 denotes an electrically conductive thin film comprising a metal oxide formed by sputtering. The conductive film 3004 is subjected to an electrification process referred to as xe2x80x9cenergization formingxe2x80x9d, described below, whereby an electron emission portion 3005 is formed. The spacing L in FIG. 1 is set to 0.5xcx9c1 mm, and the spacing W is set to 0.1 mm. For the sake of illustrative convenience, the electron emission portion 3005 is shown to have a rectangular shape at the center of the conductive film 3004. However, this is merely a schematic view and the actual position and shape of the electron emission portion are not represented faithfully here.
In the above-mentioned conventional surface-conduction electron emission elements, especially the element according to Hartwell, et al., generally the electron emission portion 3005 is formed on the conductive thin film 3004 by the so-called xe2x80x9cenergization formingxe2x80x9d process before electron emission is performed. According to the forming process, a constant DC voltage or a DC voltage which rises at a very slow rate on the order of 1 V/min is impressed across the conductive thin film 3004 to pass a current through the film, thereby locally destroying, deforming or changing the property of the conductive thin film 3004 and forming the electron emission portion 3005, the electrical resistance of which is very high. A fissure is produced in part of the conductive thin film 3004 that has been locally destroyed, deformed or changed in property. Electrons are emitted from the vicinity of the fissure if a suitable voltage is applied to the conductive thin film 3004 after energization forming.
Known examples of the FE type are described in W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8,89 (1956), and in C. A. Spindt, xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
A typical example of the construction of an FE-type element is shown in FIG. 2, which is a sectional view of the element according to Spindt, et al., described above. The element includes a substrate 3010, emitter wiring 3011 comprising an electrically conductive material, an emitter cone 3012, an insulating layer 3013 and a gate electrode 3014. The element is caused to produce a field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type element, the stacked structure of the kind shown in FIG. 2 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM type is described by C. A. Mead, xe2x80x9cOperation of tunnel emission devicesxe2x80x9d, J. Appl. Phys., 32, 646 (1961). FIG. 3 is a sectional view illustrating a typical example of the construction of the MIM-type element. The element includes a substrate 3020, a lower electrode 3021 consisting of a metal, a thin insulating layer 3022 having a thickness on the order of 100 xc3x85, and an upper electrode 3023 consisting of a metal and having a thickness on the order of 80xcx9c300 xc3x85. The element is caused to produce a field emission from the surface of the upper electrode 3023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode element makes it possible to obtain an electron emission at a lower temperature in comparison with a thermionic cathode element, a heater for applying heat is unnecessary. Accordingly, the structure is simpler than that of the thermionic cathode element and it is possible to fabricate elements that are finer. Further, even though a large number of elements are arranged on a substrate at a high density, problems such as fusing of the substrate do not readily arise. In addition, the cold cathode element differs from the thermionic cathode element in that the latter has a slow response speed because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode element is a quicker response speed.
For these reasons, extensive research into applications for cold cathode elements is being carried out.
By way of example, among the various cold cathode elements, the surface-conduction electron emission element is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of elements can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of elements, as disclosed in Japanese Patent Application Laid-Open No. 64-31332, filed by the applicant.
Further, applications of surface-conduction electron emission elements that have been researched are image forming apparatus such as image display apparatus and image recording apparatus, charged beam sources, etc.
As for applications to image display apparatus, research has been conducted with regard to such an apparatus using, in combination, surface-conduction type electron emission elements and phosphors which emit light in response to irradiation with an electron beam, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,883 and Japanese Patent Application Laid-Open (KOKAI) Nos. 2-257551 and 4-28137 filed by the present applicant. The image display apparatus using the combination of the surface-conduction type electron emission elements and phosphors is expected to have characteristics superior to those of the conventional image display apparatus of other types. For example, in comparison with a liquid-crystal display apparatus that have become so popular in recent years, the above-mentioned image display apparatus emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type elements in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A flat-type display apparatus reported by Meter et al., for example, is known as an example of an application of an FE-type element to an image display apparatus. [R. Meyer: xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahara, pp. 6xcx9c9, (1991).]
An example in which a number of MIM-type elements are arrayed in a row and applied to an image display apparatus is disclosed in the specification of Japanese Patent Application Laid-Open No. 3-55738 filed by the present applicant.
Under these circumstances, the inventors have conducted exhaustive research with regard to multiple electron source. FIG. 4 shows an example of a method of wiring a multiple electron source. In FIG. 2, a total of nxc3x97m cold cathode elements are wired two-dimensionally in matrix form, with m-number of elements arrayed in the vertical direction and n-number in the horizontal direction. In FIG. 4, numeral 3074 denotes a cold cathode element, 3072 row-direction wiring, 3073 column-direction wiring, 3075 wiring resistance of the row-direction wiring 3072 and 3076 wiring resistance of the column-direction wiring 3073. Further, Dx1, Dx2, . . . Dxm represent feed terminals for the row-direction wiring. Further, Dy1, Dy2, . . . Dym represent feed terminals for the column-direction wiring. This simple wiring method is referred to as a xe2x80x9cmatrix wiring methodxe2x80x9d. Since the matrix wiring method involves a simple structure, fabrication is easy.
In a case where a multiple electron beam source constructed using the matrix wiring method is applied to an image display apparatus, it is preferred that m and n each be a number of several hundred or more in order to assure display capacity. In addition, it is required that an electron beam of desired intensity be capable of being produced from each cold cathode element in order to display an image at a correct luminance.
In a case where a large number of matrix-wired cold cathode elements are driven in the prior art, the method adopted is to drive the group of elements on one row of the matrix simultaneously. Rows driven are successively changed over one by one so that all rows are scanned. In accordance with this method, drive time allocated to each element is lengthened by a factor of n in comparison with the method of scanning all elements successively one element at a time, thus making it possible to raise the luminance of the display apparatus.
However, when a matrix-wired multiple electron beam source is actually driven by the above-described drive method, a problem which arises is that the intensity of the electron beam outputted from each cold cathode element deviates from the desired value. This results in unevenness or fluctuation in the luminance of the display image and, hence, a decline in picture quality.
This problem will be described in greater detail with reference to FIGS. 5Axcx9c7B. In order to avoid overly complicated drawings, FIGS. 5Axcx9c7A illustrate only one row (n pixels) of the mxc3x97n pixels. Each pixel is provided to correspond to a respective cold cathode element. The farther to the right the position is taken, the more distant the position is from the feed terminal Dx of the line wiring 3072. For the sake of simplifying the description, luminance levels are represented by numerical values, the maximum value is 255, the minimum value is 0 and the intermediate values grow successively larger by 1.
FIG. 5A illustrates an example of a desired display pattern, in which it is desired that only the right-most pixel be made to emit light at the luminance 255. FIG. 5B illustrates measurement of the luminance of an image displayed by actually driving the cold cathode elements.
FIG. 6A illustrates another example of a desired displayed pattern, in which it is desired that the group of pixels on the left half of the row be made to emit no light (luminance 0) and that the group of pixels on the right half of the row be made to emit light at luminance 255. FIG. 6B illustrates measurement of the luminance of an image displayed by actually driving the cold cathode elements.
FIG. 7A illustrates another example of a desired displayed pattern, in which it is desired that all pixels of the row be made to emit light at luminance 255. FIG. 7B illustrates measurement of the luminance of an image displayed by actually driving the cold cathode elements.
Thus, as evident from these examples, the luminance of the actually display image deviates from the desired luminance. Moreover, if attention is directed toward the pixel indicated by arrow P in these Figures, it will be apparent that the magnitude of the deviation from the desired luminance is not necessarily constant.
Accordingly, an object of the present invention is to obtain a correct intensity for the electron beams produced by a multiple electron beam source having matrix-wired cold cathode elements, and to prevent a deviation in the display luminance of an image display apparatus.
When a plurality of matrix-wired cold cathode elements are driven simultaneously in one row, the drive currents in the row (=n elements) merge in the row wiring of this row. Since the junction at which merging takes place differs for each cold cathode element, there are a total of n-number of junctions on one row wire. Though the drive current which flows into each cold cathode element differs in dependence upon the desired electron-beam output value, the drive currents merge at different points so that the current which flows into each portion of the row wire is not uniform, depending upon the location. Accordingly, loss (voltage drop) produced by the electrical resistance 3075 at each portion of the row wire also is not uniform, depending upon the location. Though each cold cathode element is influenced by this loss, the manner in which this influence is received differs for each element depending upon the position at which each element is connected to the row wire. What is noteworthy here is that the loss (voltage drop) which has an influence upon a certain cold cathode element is contributed to by the drive currents of the other cold cathode elements in the same row.
In the prior art, the electron beam outputted by a cold cathode element deviates from the desired intensity owing to the loss (voltage drop) produced in each portion of the row wire. In accordance with the present invention, however, a correction is applied to the drive signals upon analyzing loss in advance. As a result, the intensity of an outputted electron beam exhibits almost no deviation from the desired value. In particular, according to the invention, loss (voltage drop) produced in row wiring is analyzed with high precision by statistically quantifying the desired output intensity of all cold cathode elements in the row. This makes highly accurate correction possible.
More specifically, according to the present invention, the foregoing object is attained by providing an electron-beam generating device comprising: a plurality of cold cathode elements arrayed in the form of rows and columns on a substrate; m-number of row wires and n-number of column wires for wiring the plurality of cold cathode elements into a matrix; and drive signal generating means for generating signals which drive the plurality of cold cathode elements. The drive signal generating means includes statistic quantity calculating means for performing a statistical calculation with regard to the externally entered electron-beam demand values, correction-value generating means for generating correction values on the basis of results of calculation by the statistic-quantity calculating means, combining means for combining the externally entered electron-beam demand values, and the correction values and means for successively driving the matrix-wired cold cathode elements on the basis of an output value from the combining means.
The present invention further provides a method of driving an electron-beam generating device having a plurality of cold cathode elements arrayed in the form of rows and columns on a substrate, as well as m-number of row wires and n-number of column wires for wiring the plurality of cold cathode elements into a matrix. The drive method comprises a statistic calculating step of performing a statistical calculation with regard to the externally entered electron-beam demand information; a correction-value generating step of generating correction values on the basis of results of calculation at the statistic calculating step; a combining step of combining the externally entered electron-beam demand values and the correction values; and a step of successively driving, row by row, the matrix-wired cold cathode elements on the basis of combined results obtained at the combining step.
In accordance with the device or drive method described above, a statistical operation is performed with regard to the electron-beam demand values and a correction is applied based upon the results of the operation. Even if the required electron-beam output pattern changes, therefore, a correction suited to the changed pattern can be applied.
In the electron-beam generating device of the present invention, the statistic-quantity calculating means includes means for calculating a sum total of one row of electron-beam demand values with regard to the externally entered electron-beam demand values.
In the drive method of the present invention, the statistic-quantity calculating step includes a step of calculating a sum total of one row of electron-beam demand values with regard to the externally entered electron-beam demand information.
In accordance with the device or drive method described above, the sum total of one row of electron-beam demand values can be ascertained, and therefore it is possible to ascertain the sum total of drive currents when the elements on one row are driven simultaneously. As a result, a correction conforming to the sum total of one row can be performed when the elements in one row are driven simultaneously.
In the electron-beam generating device of the present invention, the correction-value generating means includes means for calculating a current, which will flow into the row wires and column wires at the time of drive, on the basis of results of calculation by the statistic-quantity calculating means and output characteristic of the cold cathode elements, analyzing amount of electrical loss due to wiring resistance, deciding amount of correction for compensating for the loss and outputting the amount of correction.
In the electron-beam generating method of the present invention, the correction-value generating step includes a step of calculating a current, which will flow into the row wires and column wires at the time of drive, on the basis of results of calculation at the statistic calculating step and output characteristic of the cold cathode elements, analyzing amount of electrical loss due to wiring resistance, deciding amount of correction for compensating for the loss and outputting the amount of correction.
In accordance with the device or drive method described above, the current which flows into a row wire and a column wire at the time of drive is calculated based upon the output characteristic of the cold cathode element, and the amount of loss (voltage drop) ascribable to wiring resistance can be analyzed. Accordingly, a correction voltage necessary to compensate for the voltage drop can be determined accurately and a highly precision correction can be carried out.
In the electron-beam generating device of the present invention, the correction-value generating means includes a look-up table which stores correction quantities predetermined with regard to all cases of results of calculation capable of being outputted by said statistic-quantity calculating means.
The correction quantities stored in the look-up table in advance are correction quantities obtained by calculating a current, which will flow into the row wires and column wires at the time of drive, on the basis of output characteristics of the cold cathode elements with regard to all cases of results of calculation capable of being outputted by the statistic-quantity calculating means, analyzing beforehand the amount of electrical loss due to wiring resistance, and determining the correction quantities in advance based upon results of analysis.
In the electron-beam generating method of the present invention, the correction-value generating step includes a step of reading correction quantities out of a look-up table which stores the correction quantities predetermined with regard to all cases of results of calculation capable of being outputted at the statistic-quantity calculating step.
The correction quantities read out of the look-up table are correction quantities obtained by calculating a current, which will flow into the row wires and column wires at the time of drive, on the basis of output characteristics of the cold cathode elements with regard to all cases of results of calculation capable of being outputted at the statistic-quantity calculating step, analyzing beforehand an amount of electrical loss due to wiring resistance, and determining the correction quantities in advance based upon results of the analysis.
In accordance with the above-mentioned device or drive method, it is unnecessary to calculate a correction value whenever drive is performed.
In the electron-beam generating device of the present invention, the correction-value generating means comprises means for outputting correction quantities V1xcx9cVn calculated in accordance with the equation shown below.
In the drive method of the present invention, the correction-value generating step comprises a step of outputting correction quantities V1xcx9cVn calculated in accordance with the equation shown below.       (          xe2x80x83        ⁢                            V1                                      V2                                      V3                                      ⋮                                      Vn                      ⁢          xe2x80x83        )    =                    r        x            ·              (                  xe2x80x83                ⁢                                            1                                      1                                      1                                      1                                      …                                      1                                                          1                                      2                                      2                                      2                                      …                                      2                                                          1                                      2                                      3                                      3                                      …                                      3                                                          ⋮                                      ⋮                                      ⋮                                      ⋮                                                      xe2x80x83                                                    ⋮                                                          1                                      2                                      3                                      4                                      …                                      n                                      ⁢                  xe2x80x83                )            ·              (                  xe2x80x83                ⁢                                            I1                                                          I2                                                          I3                                                          ⋮                                                          In                                      ⁢                  xe2x80x83                )              +          Ra      ·              (                  I1          +          I2          +                      ⋯            ⁢            In                          )            ·              (                  xe2x80x83                ⁢                                            1                                                          1                                                          1                                                          ⋮                                                          1                                      ⁢                  xe2x80x83                )              +                  (                  Rb          +                      j            ·                          r              y                                      )            ·              (                  xe2x80x83                ⁢                                            I1                                                          I2                                                          I3                                                          ⋮                                                          In                                      ⁢                  xe2x80x83                )            
where the parameters are as follows:
V1xcx9cVn: correction quantities for cold cathode elements of columns 1xcx9cn in j-th row;
I1xcx9cIn: current values, to be passed through column wires of columns 1xcx9cn, calculated based upon externally entered electron-beam demand values and electron emission characteristics of cold cathode elements;
Ra: electrical resistance of extracted portion of row wiring;
I1+I2+ . . . +In: sum total of one row of externally entered electron-beam demand values (namely results of calculation by said statistic calculating means);
Rb: electrical resistance of extracted portion of column wiring;
ry: electrical resistance between cold cathode elements of column wiring;
rx: electrical resistance between cold cathode elements of row wiring;
n: total number of columns of matrix; and
j: row number (1xe2x89xa6jxe2x89xa6m).
In accordance with the above-mentioned device or drive method, an optimum correction quantity for each cold cathode element can be calculated with respect to all combinations of electron-beam demand values. This makes it possible to perform a highly precise correction. Moreover, since the wiring resistance of the column wiring is included as a parameter in the equation, an optimum correction quantity is calculated accordingly even if the row driven is changed.
Further, in the electron-beam generating device of the present invention, the correction-quantity generating means includes a first-in last-out circuit and an adder circuit.
Further, the combining means adds or multiplies together the externally entered electron-beam demand values and correction quantities generated by the correction-value generating means.
Further, in the drive method of the present invention, the correction-quantity generating step includes a step of performing operations using a first-in last-out circuit and an adder circuit.
Further, the combining step includes a step of adding or multiplying together the externally entered electron-beam demand values and correction quantities generated at the correction-value generating step.
In accordance with the above-mentioned device and method, correction values can be calculated accurately and at high speed by a simple circuit arrangement.
In the electron-beam generating device or drive method of the present invention, image information is used as the externally entered electron-beam demand values.
The above-mentioned device or drive method is ideal for use in various image forming apparatus such as an image display apparatus, printer or electron-beam exposure system.
In the electron-beam generating device of the present invention, surface-conduction electron emission elements are used as the cold cathode elements.
The above-mentioned device is simple to manufacture and even a device having a large area can be fabricated with ease.
If the electron-beam generating device of the present invention is combined with an image forming member for forming an image by irradiation with an electron beam outputted by the electron-beam generating device, an image forming apparatus having a high picture quality can be provided.
If the above-mentioned image forming apparatus has phosphors as the image forming members for forming an image by irradiation with the electron beam, an image display apparatus suited to a television or computer terminal can be provided.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.